1. Field of the Invention
The present invention relates to a phase shift mask and its manufacturing method, and more specifically to a phase shift mask of halftone type used for lithography technology and to a method for manufacturing the phase shift mask of halftone type.
2. Description of the Related Art
With increased miniaturization of circuit patterns in a semiconductor device, it is necessary to improve a photolithography in resolution which is used in a patterning process. Improvement of photomasks has made progress in improving resolution of the photolithography, and a phase shift mask of halftone type using a halftone film (semitransparent film) is known as one of the improved photomasks. Such a photomask has been suggested, for example, in U.S. Pat. No. 4,890,309.
A phase shift mask of halftone type is generally manufactured in, for example, the following process.
First, as shown in FIGS. 1A and 1B, a halftone film 2 and a shading film 3 are one after another formed on a glass substrate 1, and then, as shown in FIG. 1C, an electron-beam resist 4 of positive type is applied onto the shading film 3.
Next, a latent-image pattern is formed by means of electron-beam exposure on a part of the electron-beam resist 4 existing in a device pattern forming area 5, and the whole device pattern forming area 5 is uniformly irradiated with an electron beam which has been reduced in dose. When the electron-beam resist 4 is developed, as shown in FIG. 1D, the electron-beam resist 4 is made thin in the device pattern forming area 5 and a part of it corresponding to the latent-image pattern is opened to form a window 6.
After this, the halftone film 2 and the shading film 3 under the electron-beam resist 4 are etched by means of a wet etching or dry etching method as using the electron-beam resist 4 as a mask to make a light-transmitting pattern 7 open as shown in FIG. 1E.
Next, a part of the glass substrate 1 exposed from the light-transmitting pattern 7 is etched by means of a reactive ion etching method (RIE) to form a dented part 8 to be a phase shifter, as shown in FIG. 1F. Depth of the dented part 8 is made equal to the thickness of a glass layer to shift an incident light by 180 degrees in phase. For example, the dented part is made to be 3890 A in depth in case of forming a phase shift mask for i rays. In a step where the dented part 8 has been formed in the glass substrate 1 in this manner, the electron-beam resist 4 is removed from the device pattern forming area 5 by means of oxygen plasma and the like.
And as shown in FIG. 1G, a part of the shading film 3 existing in the device pattern forming area 5 is etched off by using the electron-beam resist 4 left on the glass substrate 1 as a mask.
Finally, when the electron-beam resist 4 is removed with a solvent as shown in FIG. 1H, a phase shift mask of halftone type is completed.
In case of using the phase shift mask in an exposure process, a light beam passing through the light-transmitting pattern 7 of the phase shift mask is little damped while a light beam passing through the halftone film 2 is damped to a degree of 10% of it. And the light beam passing through the light-transmitting pattern 7 and the light beam passing through the halftone film 2 result in being different by 180 degrees in phase from each other thanks to action of the dented part 8.
The reason why a shading film 3 is disposed around the device pattern forming area 5 is as follows. In the case of performing an exposure process as using a phase shift mask and a stepper, if such a part to be repeatedly exposed as a boundary (scribing area) between plural device areas or an alignment mark necessary for aligning a mask is covered with a halftone film 2, lights passing through the halftone film 2 are accumulated in the area and finally the area results in being in the same state as it has been irradiated with little-damped light beams. From this reason, such an area as around the device pattern forming area 5 is covered with the shading film 3. However, since a side etching S as shown in FIG. 2 may appear on the halftone film 2 between the shading film 3 and the glass substrate 1 when patterning the shading film 3, there may be an undesirable problem that a pattern of the halftone film 2 varies in shape. Additionally, it takes a long time to make an inspection for side etching in addition to the primary inspection for pattern defects in case of inspecting a finished phase shift mask by means of a surface inspection apparatus.